Tin (II) (stannous) ions (i.e., stannous chloride) are added to oral care compositions to deliver multiple benefits including, for example: anti-microbial effects, control of breath malodor, control of dental plaque growth and metabolism, and reduced gingivitis. However, oral care compositions containing stannous chloride, especially in combination with thickening agents such as carboxymethyl cellulose (CMC), can suffer from poor rheological properties.
One of the main reasons for the problem is that Sn2+ ion is prone to oxidation towards Sn4+ causing the oral care composition to exhibit an unacceptably low viscosity. If a formulation routinely decreases in viscosity, such oral care composition can lack phase stability and tends to undergo phase separation over time or physical degradation of the structure. The problem is especially noticeable at the tip of the dispenser due to higher levels of oxygen penetrating the container from the exterior and reacting with the oral care composition closest to the tip.
As a result, consumer dissatisfaction will likely result as the oral care composition nearest the tip will be dispensed first from the dispenser and appear watery as the liquid has separated from the body of the composition. Various formulation approaches have been tried to stabilize stannous chloride containing oral care composition. PCT Publication No. WO2008/41055 (P&G) discloses stannous chloride containing oral care compositions. The composition examples include carboxymethyl cellulose (CMC). PCT Publication No. WO2010/114546 (Colgate-Palmolive) discloses dentrifice compositions comprising polysaccharide thickeners having xanthan gum and hydroxyethyl cellulose (HEC) for decreasing the viscosity of the toothpaste.
Despite the foregoing, there is still a need for stannous ions containing oral care compositions having improved phase stability and/or shelf-life stability over time (greater than 4 months to 24 months), at ambient conditions.